An electrical cable includes an insulation material between a conductor and the closest electrical ground, thus preventing an electrical fault. Generally, the insulation may be made of a crosslinked or non-crosslinked polymeric composition with electrical insulating properties chosen, for example, from: polyolefins (homopolymers or copolymers of various olefins), ethylenically unsaturated olefin/ester copolymers, polyesters, polyethers, polyether/polyester copolymers, and blends thereof.
Examples of polymers suitable for electrical cable insulation are polyethylene (PE), in particular linear low-density PE (LLDPE); polypropylene (PP); propylene/ethylene thermoplastic copolymers; ethylene/propylene rubbers (EPR) or ethylene/propylene/diene rubbers (EPDM); natural rubbers; butyl rubbers; ethylene/vinyl acetate (EVA) copolymers; ethylene/methyl acrylate (EMA) copolymers; ethylene/ethyl acrylate (EEA) copolymers; ethylene/butyl acrylate (EBA) copolymers; ethylene/α-olefin copolymers, and the like.
During electrical cable manufacture, the insulation material, as well as other material layers, are generally placed on the cable by extrusion. In an extrusion process, pellets comprising plastic resin or other materials are first loaded into a hopper and then fed into a thermoregulated barrel of an extruder. Within the barrel, the pellets are heated to the point of melting and moved along the barrel by the action of at least one continuously revolving screw. At the end of the barrel, the molten plastic is forced out through a die that is cast in the shape of the finished product to be obtained.
For purposes of this description, “pellets” refers generally to small particulates or granules of a material. Although “pellets” may connote an elliptical shape in some contexts, the extrusion pellets addressed in this description are not limited to a particular geometry. Consequently, pellets may be cylindrical, spherical, oblong, rectangular, square, or any other shape.
Devices are often employed to improve the condition of the resin pellets before extrusion of the insulation takes place. For example, devices may be used for removing residual resin material from a batch of the granular compound just before the pellets enter the extruder. This residual material is called “fines.” For purposes of this description, “fines” are substances of the same material as the resin pellets but not having a granular or pelletized form. Also called “fluff” or “streamers,” fines often are in the shape of strings, hair, or powder. Fines can clog machinery and degrade the throughput of the extruder. Although fines also can degrade the quality of the extruded polymer, for purposes of this description fines are not considered contaminants within the batch of material.
Devices called dedusters are conventionally used to remove fines and dust from a batch of granular resin material. When extruding electrical cable insulation, dedusters are typically employed at the input feed to the extruder. U.S. Pat. No. 4,631,124 is exemplary in describing the operation of a conventional deduster. It discloses a deduster that employs gravity to feed dust and impurity laden particulate material through a linear kinetic energy cell. The cell generates an electric field to neutralize the static electric charges that causes the dust to adhere to the particulate material. With the static electric charge neutralized, the dust (and fines) can be separated by an air flow substantially transverse to the particle flow. Removal of the impurities can be accomplished by pressurized air or a vacuum.
The remaining pellets can be extruded to form insulation for an electrical cable, for example. An electrical cable includes a cable core at its interior. In the present description, the term “cable core” indicates a semi-finite structure comprising a conductor and at least one layer of electrical insulation placed in a position which is radially external to said conductor. More particularly, when considering a cable for the transport or distribution of medium/high voltage electrical power, the cable core may further comprise an internal semiconductive layer (i.e. a conductor shield), an external semiconductive layer (i.e. an insulation shield), and a metal screen. The internal semiconductive layer is located in a position radially external to the conductor. The external semiconductive layer is located in a position radially external to the insulation. The metal screen is in a position radially external to the insulation shield.
To avoid stress-concentrating irregularities in the cable core, the conductor shield, the insulation, and an insulation shield may be applied simultaneously by co-extrusion. Generally, this is accomplished by a triple-output extrusion head in conjunction with automated scanning devices to monitor each layer for thickness and concentricity directly after the layers are applied. By monitoring the extrusion process, automatic controls may correct any variation in thickness or concentricity.
After the insulation is extruded onto the conductor, it is cured. With XLPE, for example, the curing process causes carbon atoms to link to adjacent polyethylene chains, resulting in cross-linking. Cross-linking improves the thermo-mechanical properties of the insulation. After the cable is cured in the tubes, it is taken up on large reels.
The insulation in an electrical cable may degrade for a number of reasons. For example, polyethylene is susceptible to degradation due to partial discharge that may in turn lead to “water treeing.” Water treeing is the phenomenon whereby small tree-like voids form and grow in the insulation and may fill with water. If a tree grows large enough in the insulation, electrical breakdown, and thus cable failure, can occur between the conductor and an electrical ground.
In electrical cables, failure mechanisms, like water treeing, are more likely to occur when imperfections exist in the insulation layer. Imperfections in the insulation often result from contaminants within the batch of resin material used to extrude the insulation.
For purposes of this description, “contaminants” generally refers to particles having characteristics that are undesirable for the material being extruded. Contaminants might include, for example, metal, dirt, or just about any material different from the pellet material. However, some fines may be considered contaminants as well for purposes of this description. For example, as opposed to “clean fines,” fines that have contaminants attached to them (known as “dirty fines”) and fines that have become thermally degraded (known as “amber fines”) often are discolored and can contaminate the resin batch.
Conventionally, it has been thought that most extrusion imperfections are caused by contaminants embedded within the resin pellets. Contaminants that are embedded within the pellets, generally referred to as defective pellets, can be difficult to identify and to separate from desired pellet materials.
Several approaches are known for generally separating defective pellets from desired pellets. In these approaches, “defective pellets” often include other deficiencies beyond just having contaminants embedded within them. For example, for some applications, extrusion pellets may be defective because they include air bubbles, contain material impurities, have differing geometries, or have differing colors. In general, conventional equipment for removing defective pellets is effective in removing pellets having embedded contaminants.
The first step in many of these approaches is to first feed the pellets onto a conveying belt. An endless driving belt may, for example, constitute the conveying belt. Thereafter, defective pellets that are conveyed on the conveying belt can be detected by using some sort of separation device.
Further details for separating defective discrete materials are described in EP 0 705 650 A2. In this application, a grain sorting apparatus comprises a conveyor belt mechanism for conveying grains. The grains are fed by a feed mechanism onto a conveying surface separately from each other at an upstream region with respect to a conveying direction. The grains are discriminated and sorted by a discriminating mechanism and a sorting mechanism when dropping from the downstream end along a predetermined path.
Another approach to material sorting is described in WO 99/37412. This application includes an arrangement for sorting pellets, comprising a transportation device for feeding the pellets. The device also includes a first container for faultless pellets fed over the end portion of the transportation device, a second container for defective pellets, a detector for detecting defective pellets and a sorting device for feeding any defective pellets detected to said second container.
In conventional sorting machines, the step of sorting defective pellets often occurs based on a comparison of their external appearance with a predetermined criteria using a light beam. Sorting based on the external appearance has some limitations, however, because accurate detection must be evenly carried out over the entire pellet. If defects exist inside a pellet due, for example, to an air bubble, then a sorting process limited to external criteria may not be thorough. Also, shadows and reflections from the light beam may be erroneously construed as defects in the external appearance. Consequently, this type of measurement is often very costly and complex.
U.S. Pat. No. 6,355,897 discloses an alleged improvement to pellet sorting using a device that includes a light detector arranged over a transparent pellet transport track. A light source is arranged on the opposite side of the track. The detector provides a measurement of a received light intensity, and if the measured intensity is lower than a predetermined threshold value, it can be assumed that a defect is present. The pellet containing the defect is then sorted out by actuating a sorting device. In order to obtain a high precision detection, light is distributed evenly from all directions, including ambient light.
In U.S. Pat. No. 5,201,576, a shadowless illumination system is disclosed that may include a spherical chamber having a chamber entrance opening and a chamber exit opening. The inside surface of the spherical chamber may be coated with highly reflective flat white paint. A clear rigid plastic cylindrical tube may be positioned in the spherical chamber between the chamber entrance and exit openings. A circular fluorescent ring lamp may be positioned inside the spherical chamber to form an annulus around the tube. The lamp and the white inside surface of the spherical chamber may provide shadowless illumination for articles that are dropped or otherwise projected through the tube. The articles may be inspected as they pass through the tube by at least two video inspection cameras that view opposite sides of the articles through respective viewing openings.
Applicant has noticed that these prior arrangements for filtering or cleaning resin pellets before extruding polymer products have proved to be insufficient to attain a high quality product. In particular, Applicants have noticed that upwards of 95% of the contaminants in a batch of resin material for extruding electrical cable insulation are loose particles, with 5% or less of the contaminants being embedded in the pellets. These loose contaminants may include particulate of just about any material, including insects, paper, fabric, metal, dirty fines, and amber fines. The presence of these loose contaminants mixed with the resin pellets complicates the cleaning process. While conventional pellet sorting machinery is often effective at removing pellets having embedded contaminants, they are less effective in removing loose particle contaminants, especially those that are small in size relative to the pellets.
Moreover, Applicant has observed that existing systems for removing pellets containing embedded contaminants do not sufficiently account for fines when sorting pellets. While some devices do provide for airflow around the detection device in order to move fines away from the pellets, Applicant has noticed that the airflow is insufficient to remove a significant amount of the fines. This situation leads to a build-up of fines that must be cleaned out often, creating an obstacle to continuous operation of the device.